1. Field of the Invention
The present invention relates to a discharge abnormality detection device and method used in a wire bonding apparatus which forms a ball at the end of a wire by means of a discharge.
2. Prior Art
In a wire bonding apparatus, as shown in FIG. 7, a wire 2 is held by a clamper 1, and a ball is formed on the tip end of the wire 2 using a high voltage generating device 5 that applies a high voltage across a discharge electrode 4 and the tip end of the wire 2 passing through a capillary 3. The high voltage generating device 5 includes a constant-current power supply circuit, and the discharge voltage is detected by a voltage detection part 6.
Example discharge abnormalities are an improper gap between the wire 2 and the discharge electrode 4, contamination 2a on the surface of the wire 2 and contamination 4a of the surface of the discharge electrode 4 as shown in FIG. 8, and interference with the discharge that is caused by gas currents as shown in FIG. 9.
FIG. 9 is an explanatory diagram of the essential structural elements of Japanese Patent Application Pre-Examination Publication (Kokai) No. 7-147297.
In FIG. 9, a gas 22 such as air or some other gas is blown onto the bonding horn 20 (which holds a capillary 3 at one end thereof) from horn cooling pipes 21 so as to prevent thermal expansion of the bonding horn 20. If the gas currents 22a interfere with the electric discharge of the discharge electrode 4, the direction of the discharge is changed, or the discharge may be blown out, thus causing discharge abnormalities.
In Japanese Patent Application Pre-Examination Publication (Kokai) Nos. 1-256134, 5-36748 and No. 7-37930 disclose a formation of a gas atmosphere in the discharge region between the wire and the discharge electrode. In these cases, when the flow velocity of the gas supplied to the discharge region is too high, discharge abnormalities also occurs.
Various states of the tail of the wire 2 protruding from the tip end of the capillary 3 during the ball formation are shown in FIGS. 3 and 4.
FIG. 3(a) shows an improper state in which the wire 2 is not extended out from the tip end of the capillary 3. In FIG. 3(b), the wire 2 contacts the discharge electrode 4, which is another type of improper state.
FIG. 4 shows the different types of improper states in which an unnecessary length of the tail of the wire extends from the tip end of the capillary 3 during ball formation. In FIG. 4(a), the tail is bent away from the discharge electrode 4, so that the discharge gap between the tip end of the wire 2 and the discharge electrode 4 is too wide, thus causing the tip end of the wire 2 to be positioned outside the normal detection range 7. In FIG. 4(b), the tail is bent toward the discharge electrode 4, so that the discharge gap between the tip end of the wire 2 and the discharge electrode 4 is too narrow, thus causing the tip end of the wire 2 to be positioned outside the normal detection range 7.
To the contrary, FIG. 4(c) shows a proper state of the tail of the wire; and in the proper state, the tip end of the wire 2 is positioned inside the normal detection range 7.
FIG. 5 shows the discharge voltage values detected by the voltage detection part 6 shown in FIG. 7. The states shown in the FIGS. 3(a) and 4(a) are treated as open detection, while the states shown in FIGS. 3(b) and 4(b) are treated as short detection.
A discharge voltage in the normal state shown in FIG. 4(c) will be described first. As seen from the right-end column in FIG. 5, when the discharge command 8 is initiated, a large insulation breakdown voltage Vpc.sub.4 is generated in order to break down the air (insulation) between the tip end of the wire 2 and the discharge electrode 4; and after this insulation breakdown, a discharge maintenance voltage Vc.sub.4 is generated.
However, when there is no tail (i. e., in cases where the wire 2 is not extended out of the tip end of the capillary 3) as shown in FIG. 3(a), the discharge maintenance voltage Va.sub.3 is the same as the insulation breakdown voltage Vpa.sub.3 as shown in the left-end column. Furthermore, when the wire 2 is in contact with the discharge electrode 4 (i. e., in cases where the wire 2 shorts out) as shown in FIG. 3(b), no electric discharge occurs, so that the discharge maintenance voltage Vb.sub.3 is an extremely small voltage.
As seen from the above, the magnitudes of the respective voltages (discharge maintenance voltages) after a fixed time has passed following the initiation of the discharge can be expressed by Equation 1 below; and magnitudes of the respective insulation breakdown voltages can be expressed by Equation 2 below. EQU Va.sub.3 &gt;Va.sub.4 &gt;Vc.sub.4 &gt;Vb.sub.4 &gt;Vb.sub.3 [Equation 1] EQU Vpa.sub.3 &gt;Vpa.sub.4 &gt;Vpc.sub.4 &gt;Vpb.sub.4 &gt;Vpb.sub.3 [Equation 2]
Accordingly, detection of improper tail states are accomplished by examining changes in the discharge maintenance voltages Va.sub.3, Va.sub.4, Vc.sub.4, Vb.sub.4 and Vb.sub.3 which are shown in FIG. 5 after a fixed time has passed following the initiation of the discharge.
In the meantime, the relationship between the discharge maintenance voltage and the discharge gap between the tip end of the wire 2 and the discharge electrode 4 is as shown in FIG. 6. As seen from FIG. 6, in the case of the open detection and short detection shown in FIGS. 3(a) and 3(b), a large difference appears in the discharge maintenance voltages Va.sub.3 and Vb.sub.3 ; thus detection is easy. However, in the case of the discharge maintenance voltages Va.sub.4 and Vb.sub.4 involved in bending of the wire 2 as shown in FIGS. 4(a) and 4(b), there is almost no difference from the discharge maintenance voltage Vc.sub.4 shown in FIG. 4(c); as a result, the detection of discharge abnormalities is difficult.
Furthermore, when discharge interference caused by contamination of the wire surface, contamination of the surface of the discharge electrode or gas currents, etc. occurs, a discharge fails to be generated, resulting in a voltage such as Va.sub.3 as show in FIG. 5.